1. Technical Field
The present invention relates to laser length measuring instruments and more particularly to a laser length measuring instrument for measuring the displacement of a moving object by counting up interference fringes according to interference wave signals detected by an interferometer in such a manner as to prevent count errors from accumulating to ensure high precision measurement.
2. Prior Art
Laser length measuring instruments are capable of measuring the displacement of moving objects with resolution of the order of wavelength by utilizing the interference phenomena of laser beams. FIG. 1 shows the basic configuration of an interferometer in a laser length measuring instrument.
In FIG. 1, numeral 10 denotes an interferometer in which a laser beam 1a generated from a semiconductor diode as a laser beam source (LD) 1 is received by a beamsplitter 2 where it is divided into a measuring beam m and a reference beam r. The measuring beam m is reflected from a measuring mirror 4 fitted to a moving object 5 and returns to the beamsplitter 2. The reference beam r is reflected from a reference mirror 3 fixed at a predetermined position in the interferometer 10 and passes through the beamsplitter 2 where it is combined with the measuring beam.
Since a laser beam has coherence in general, the beam synthesization results in the generation of an interference wave 6 as stated above. Given that the wavelength of the laser beam 1a is .lambda., the pitch of interference fringes of the interference wave 6 is .lambda./2.
While the measuring mirror 4 remains stationary, the interference wave 6 also stands still. If the measuring mirror 4 moves (because of the moving object 5), the interference wave also moves accordingly. An interference wave signal 6a in the form of an electric signal which changes with time as the object 5 moves can thus be obtained by applying the interference wave 6 to a light receiver 7.
By applying the interference wave signal 6a to a comparator to examine the signal with the base level as a reference, the interference wave signal 6a can be converted into a corresponding pulse. The number of pulses of the pulse waveform is proportional to the displacement .delta.x of the measuring mirror 4. Therefore, the displacement .delta.x of the moving object 5 can be measured with resolution corresponding to the wavelength (=.lambda./2) of the interference wave signal 6a by counting the pulses.
The precision of electronic parts such as semiconductors is being increasingly improved. As a result, laser length measuring instruments for use in manufacturing or inspecting electronic parts are required to be capable of measuring a displacement less than what is of the order of wavelength of an ordinary laser diode. Although the direction in which the object 5 moves has been left unidentified in the description given above, even that direction is desired to be determinable. The present applicant has made an invention that can meet such requirements and applied for a patent (Japanese Patent Laid-Open No. 35303/1989). This invention has substantially the same contents as shown in FIGS. 2(a), (b). In FIG. 2(a), numeral 11 refers to an interferometer, 12 to an interference wave signal generating circuit, and 13 to an interference fringe counting circuit.
The interferometer 11 has a polarizing beamsplitter 8 (corresponding to the beamsplitter of FIG. 1 with the omission of the reference and the measuring mirror 3, 4 in FIG. 2) and causes a linear polarizing laser beam 1b produced by a laser beam source (not shown) to be incident on the polarizing beamsplitter 8. As in the case of FIG. 1, the polarizing beamsplitter 8 divides the laser beam 1b into a measuring beam and a reference beam r; these beams are respectively reflected from the measuring mirror 4 and the reference mirror 3 before being returned to the polarizing beamsplitter 8. At this time, the measuring beam m and the reference beam r are incident on a .lambda./4 plate 9 without interfering with each other as their polarizing directions are different.
While passing through the .lambda./4 plate 9, each beam is converted into a circularly polarized light wave, whereby interfering waves of the measuring and the reference beam m, r are generated. The interference wave is divided by two nonpolarizing beamsplitters 101, 102 into three. The divided interference waves are passed through linear analyzers 111, 112, 113 in the directions which differ in phase by .pi./2 with respect to the luminous planes and sent to light receivers 71, 72, 73, respectively. As a result, interference wave signals corresponding to the interference waves detected by the respective linear analysers 111, 112, 113 are generated in the respective light receivers 71, 72, 73.
On receiving the three interference wave signals differing in phase by .pi./2, the interference wave signal generating circuit 12 makes one of the signals a reference signal and the other two signals comparative signals, and applies these signals to subtracter circuits 121, 122 as shown in FIG. 2(a). The subtracter circuits 121, 122 consequently output two interference wave signals e.sub.1, e.sub.2 differing in phase by .pi./2 as shown in FIG. 2(b).
As shown in FIG. 2(b), the interference wave signals e.sub.1, e.sub.2 become sinusoidal wave interference signals e.sub.1, e.sub.2 e.sub.2 =sinusoidal wave differing in phase by +.pi./2 from the signal e.sub.1).
The signals e.sub.1, e.sub.2 are subsequently applied to the interference fringe counting circuit 13. The interference fringe counting circuit 13 determines the polarities of the signals e.sub.1, e.sub.2 to be + or - with the base level as a reference value. The results thus determined are shown with +, - under the signals e.sub.1, e.sub.2 in the respective intervals I-IV.
The interference fringe counting circuit 13 determines the direction of movement by reference to the relation between the polarities of both signals and produces moving-direction signals. As shown in FIG. 2(b), there are two points at which the polarity inversion of each interference signal occurs every period. The interference fringe counting circuit 13 detects zero-cross points as polarity inversion points of the signals e.sub.1, e.sub.2, for instance, and counts the pulses thus detected by means of a counter. In this way, the displacement .delta.x of the moving object 5 can be measured with a resolution of .lambda./8 with respect to the wavelength of the laser beam.
The present applicant has also made an invention for further resolution improvement and applied for a patent (Japanese Patent Laid-Open No. 184402/1989). In this invention, the number of beamsplitters of the interferometer shown in FIG. 2(a) is increased to three, instead of the two shown (101, 102), to divide the interference wave into four. These interference waves are passed through respective analyzers whose polarizing planes differ by .pi./4 to cause the generation of interference wave signals differing in phase by .pi./4. These four interference wave signals are, as shown in FIG. 2(c), sinusoidal wave interference signals e.sub.a, e.sub.b, e.sub.c, e.sub.d sequentially differing in phase by .pi./4. As there are two polarity inversion points every period for each, the resolution with respect to the displacement .delta.x of the moving object 5 can be raised to .lambda./16 by detecting the polarity inversion points likewise and counting the pulses thus detected.
As is obvious from the principle, it is possible to improve the resolution by increasing the number of beamsplitters to increase the number of divided interference waves and to decrease the phase difference therebetween. However, the use of the optical method for dividing the interference wave by means of the beamsplitter tends to make the optical system configuration extremely complicated and therefore to render the adjustment of the optical axis and the like difficult.
Moreover, the frequency of the interference wave signal is proportional to the speed of the moving unit; the higher the speed, the higher the frequency becomes. The frequency becomes proportionally higher when the resolution is increased. As the frequency rises, the occurrence of a measurement error is induced under the influence of noise and the like. The conventional measuring system has the further disadvantage of causing errors in measuring data to be accumulated when a measurement error occurs because the pulses corresponding to the interference fringes are additive. As a result, frequency of error in the pulse for detecting the interference fringe increases when the resolution is improved and this results in decreasing the reliability of the measured value.
The principle in the semiconductor laser length measuring instrument using an interferometer relies on causing the generation of the interference waves through the interference of the measuring beam with the reference beam. The interference wave then becomes most clarified when both the beams have the same wavelength. However, the laser beam generated by a semiconductor laser is such that the frequency fluctuates and becomes irregular; consequently, it has a certain breadth with respect to the central wavelength generated. If the interference wave is divided into pieces, the relation of one to another tends to become unclear and resolution becomes difficult to improve.